A machine component having a fluid passage is liable to experience stress concentration at the ends of the fluid-conveying tube and regions of the tube where its diameter changes radically so that fatigue fractures caused by fluid pressure fluctuation may become an issue.
A common rail is a tubular component which is provided between a pump for pumping diesel fuel and injectors in a diesel engine accumulator fuel-injection system, so as to store fuel under pressure. FIG. 1 is a schematic cross-section of a common rail 1. A rail hole 5, which is the main pipe of the common rail 1, functions to store pressurized diesel fuel. The rail hole 5 is provided with a number of branch holes 6 which open to the vertical direction, and the diesel fuel is pumped through the branch holes 6 to associated injectors. The rail hole 5 has an inside diameter “d1” of about 10 mm, and the branch holes 6 have an inside diameter “d2” of about 1 mm. During engine operation, diesel fuel is periodically pumped and the pressure of the diesel fuel in the common rail 1 therefore varies periodically. In the course of the periodic pressure variation, the rail hole 5 and the branch holes 6 shown in FIG. 1 experience periodic variation in circumferential tensile stress. FIG. 2 shows an enlarged view of the boundary peripheral region between the inside surface of the branch hole 6, which is an opening peripheral zone of the branch hole 6, and the inside surface of the rail hole 5. Among the different sectors of the opening peripheral zone of the branch hole 6, the zones 7 near the opposite ends of the diameter of the branch hole 6 parallel to the longitudinal direction of the rail hole 5 are zones where the tensile stresses of the two holes 5 and 6 are added. Therefore, these zones 7 experience greater tensile stress than other zones and tend to undergo fatigue fracture owing to internal pressure variation. Improvement of fatigue strength against internal pressure variation (internal pressure fatigue strength) would enable high-pressure injection of fuel and is therefore desirable from the aspects of clean exhaust gas and fuel efficiency.
Up to now, improvement of fatigue strength has generally been approached by using high-strength steel to increase the fatigue strength of the common rail. However, this method degrades the formability and workability of the common rail owing to the high strength of the steel and increases cost in proportion to steel performance enhancement. In response to these problems, Patent Document 1, for example, teaches an invention that replaces the conventional method of producing a common rail by monolithic forging and mechanical processing with a method of producing a welded common rail by liquid phase diffusion bonding. Further, Patent Document 2 teaches an invention related to steels suitable for liquid phase diffusion bonding that do not require controlled cooling during bonding. However, the steel taught by this patent reference has a tensile strength of about 600 MPa and as such is deficient in strength for use at 1500 atm or even 2000 atm and higher pressure common rails needed to realize the high-fuel efficiency aimed at in recent years. Although the steel strength can be markedly improved by heat treatments and the like, this approach makes processing difficult and greatly increases production cost. In addition, in the case where the processing exposes oxides and/or inclusions such as MnS, Al2O3, CaO and the like at the surface of the maximum principal stress regions, the oxides and/or inclusions become fatigue fracture starting points during internal pressure application. This seriously impairs stable production of high-strength common rails and is a problem that cannot be overcome.
Moreover, attempts have not been limited to the ordinary method of increasing steel strength. Regarding common rail strength, for example, Patent Document 3 and Patent Document 4 teach methods of mitigating stress concentration by using fluid polishing or coining treatment to chamfer the edges of the branch hole opening region edges. Improvement of fatigue strength by imparting compressive stress has also been studied. Laser peening is one technology that has been developed. In this technology, a liquid or other transparent medium is provided on the surface of a metal object and a pulsed laser beam of high peak power density is directed onto the metal surface. Then, utilizing the expansion reaction force of the plasma produced thereat, residual compressive stress is imparted near the surface of the metal object. A method utilizing this technology is taught by Patent Document 5, for example. A laser beam can be transmitted even to narrow regions such as the inner surface of the rail hole and the inner surfaces of the branch holes of the common rail, thus, laser peening is currently the only method available for imparting high compressive stress in the vicinity of the branch hole openings of the common rail. Thus, as can be seen from Patent Document 6, effective methods for applying laser peening to common rails are being explored.
While the method taught by Patent Document 6 enables considerable improvement of common rail fatigue strength, it has the following drawbacks from the aspects of system and effect. When the laser beam is directed onto the sample surface during laser peening, the surface layer at and around the laser spot melts and resolidifies, so that the surface layer near the laser spot often declines in compressive stress. A known way to avoid this problem is to provide a sacrifice layer for absorbing the laser beam. However, a complex system is required for setting the sacrifice layer at the branch hole opening regions of the common rail. It is therefore desired to avoid this process from the viewpoint of cost and productivity.
Patent Document 5 discloses methods for removing heat affected zones. The methods including a process of producing a laser-beam-controlled electric discharge between a laser-beam-exposed surface and an electrode installed facing near the laser-beam-exposed surface, and a process of conducting electrolytic polishing between an electrode installed facing the laser-beam exposed surface and near the surface irradiated with the laser beam, using an electrolyte liquid as a transparent liquid provided in contact with the laser-beam-exposed surface. However, accurate and stable processing to the desired shape is difficult with these methods because the influence of the laser-beam irradiation is great. The methods are therefore unsuitable for industrial manufacture of common rails. As reported in Patent Document 6, the aforesaid problem of a decline in compressive stress is mitigated by increasing the superimposed area of the adjacent pulsed laser beam spots. However, in order to boost the effect of improving the common rail fatigue strength to a still higher level, it is necessary to maximize the compressive stress near the surface layer, therefore, a different approach is desired.